Energy Balances of Diploid, Triploid, and Hybrid Grass Carp

Grass carp Ctenopharyngodon idella and genetic derivatives are widely used as biological control agents for aquatic macrophytes. The results of bioenergetic comparisons of three types of commercially available grass carp are reported. Empirical relationships were developed for consumption rate, metabolic rate, and assimilation efficiency as functions of temperature and size. Summary energy balances are presented for each type of fish. Diploid and triploid grass carp had similar standardized energy balances: 12–13% of the energy ingested was used in metabolism, 74% was egested or excreted, and 13–14% went into growth. Hybrid grass carp (female grass carp x male bighead carp Hypophthalmichthys nobilis) metabolized a greater proportion of the energy intake (16%) and egested 81%, leaving only 3% available for growth; they also ingested food at a significantly lower rate. Thus, intergeneric triploid hybrids appear to be energetically handicapped. Their low consumption rate and reduced assimilation efficiency, coupled with elevated metabolic costs, leave little energy available for somatic growth. High rates of growth of diploid and triploid grass carp are attributed to their “low efficiency-high volume” energetic strategy, which is quite successful as long as food supplies allow them to feed ad libitum.